This invention relates to a monopole electromagnetic antenna having one or more radiating elements in the form of elongated conductors extending from a support, such as a plurality of wires secured about a mast, and being operative in a frequency band including frequencies wherein the antenna is electrically short, less than one-quarter wavelength.
Such an antenna is advantageous because of mechanical simplicity which facilitates installation, removal, and transportation of the antenna. For example, the antenna is readily transported by a truck, may be mounted to the truck, and is readily positioned to provide an azimuthal radiating pattern. The antenna may be deployed as part of a communication link.
A problem arises, particularly at the lower frequencies of the band wherein the antenna is electrically short. For example, in the case wherein the radiating elements comprise a set of wires, the radiating elements exhibit resonant behavior and capacitive reactance which narrow the bandwidth, introduce losses in a matching network between a transmitter and the antenna, and produce an antenna input impedance which varies rapidly with frequency. Also, there is relatively low radiation resistive loading resulting in a low power gain. Therefore, the use of such an antenna has been restricted to situations in which the output stage or matching network of a transmitter connected to the antenna is tuned to operate over a relatively narrow bandwidth. Where it has been necessary to employ the antenna in a wider band situation, such as a situation involving a scanning of the signal frequency, it has been necessary to rematch the antenna at each frequency of operation. This limits the rapidity of frequency shifting. The foregoing comments relating to the use of a transmitter with the antenna apply also to the use of a receiver with the antenna because the antenna operates in reciprocal fashion and produces the same antenna characteristics both for transmission and reception.